1. Field of the Invention
This invention relates to the preparation of optically transparent electrode layers and more particularly to the preparation of such layers which are arranged in a pattern on a substrate for use in electro-optical displays.
2. Description of the Prior Art
In various types of electro-optical displays, for example, liquid crystal displays, a dielectric medium is interposed between a continuous electrode layer and a transparent patterned electrode layer. By electrically energizing selected portions of the patterned transparent layer, an electric field is applied to the dielectric medium between the energized portion of the patterned electrode and the continuous electrode. This electric field can modify the properties of the dielectric medium, for example of a liquid crystal layer, so that a display corresponding to the energized areas is made visible through the transparent patterned electrode.
The transparent patterned electrically conducting electrodes are generally supported on transparent support plates or substrates. The electrodes are usually prepared by first coating one surface of the support, for example by vapor deposition or sputtering in vacuo, with a continuous thin film of a transparent, electrically conductive material, for example, semiconductive metal oxides such as In.sub.2 O.sub.3 and/or SnO.sub.2, and then selectively removing portions of the deposited film by etching to produce a patterned electrode. As freshly deposited, that is without heat treatment, the vapor deposited or sputtered films can be etched relatively easily with acids. In order to confine the etching to selected areas, the film is covered with a photoresist, which is a layer of photopolymerizable material. The photoresist is then polymerized imagewise by exposure to actinic radiation. The unpolymerized resist in the unexposed areas can then be removed, for example by a suitable solvent, to form the windows of an etching mask. Subsequently, the uncovered areas of the transparent electrically conductive film are etched away, and the photoresist layer is removed from the covered areas of the transparent electrode film, which remain on the substrate. This photoresist etching process makes it possible to prepare very complex patterned electrodes and those having very fine lines. However, it is very expensive and/or time-consuming.
Transparent patterned electrodes can also be prepared by vapor-depositing the electrically conductive material for the transparent electrode film in the desired pattern onto the appropriately masked substrate. The areas of the substrate which are not to receive an electrode film are covered either with a metal mask placed in contact with the substrate or by a corresponding mask pattern which is printed onto the substrate (see, for example, German Auslegeschrift No. 2,322,826). Masking using a metal mask presents problems since it is very difficult to attain the necessary close contact between the mask and the substrate and there is a very great risk of unsharp masking with a corresponding poor definition of the vapor-deposited pattern. Printing a mask on the substrate before the film is vapor-deposited also presents problems, since the printed masking layer produces so-called marginal shadows and can thus impair the definition of the electrode pattern. Moreover, the printed mask first must be dried before the vapor-deposition step and must be removed after vapor deposition, for example by washing, burning off, or brushing. The removal is made more difficult by the layer of vapor deposited material which covers both the masked and unmasked areas.
Therefore, a need has continued to exist for a method of producing patterned transparent electrodes on substrates which avoids the problems introduced by the known methods of etching or masking.